1. Field of Invention
This invention relates to resistive digitizers and in particular noise filtration and coordinate voltage measurement for four and five wire digitizers.
2. Description of Related Art
Resistive digitizers when placed on an LCD or CRT screen pick up noise that is superimposed on the coordinate voltage that is to be read. This type of digitizer has two resistively coated surfaces that come into contact when pressed. The noise cannot be easily filtered because the different terminals are multi-function and see different voltages at different times. For example, in a four wire digitizer a terminal can be used to connect power to the digitizer panels or to read the coordinate voltage of the other panel. A filter one might use on a power terminal would have time constant implications when switching between functional use of the terminal, and a filter one might use in a signal voltage would have a voltage drop implication when used as a power terminal
In U.S. Pat. No. 4,306,110 (Nelson et al.) is described an apparatus for determining planar coordinates. A four wire digitizer is discussed with the un-powered plane providing the location voltage through a high impedance conditioning circuit to a sample and hold circuit. As power is alternated between digitizer planes, un-powered plane is changed to read X-plane coordinate voltages and then Y-plane coordinate voltages. These voltages are connected to different inputs of the high impedance conditioning circuitry and further connected to sample and hold circuits at the output of the high impedance and conditioning circuits.
In U.S. Pat. No. 4,484,026 (Thornburg) a touch tablet is shown for entering data into a computer. The touch tablet is made of two resistive sheets in parallel and oriented orthogonal to each other. Conductive strips on two ends of each resistive sheet and having orthogonal orientation between sheets are electrically connected to the resistive material of the sheet. A voltage is applied between the strips on one resistive sheet, and a coordinate location is read from the un-powered sheet. By connecting the output amplifier to both strip on the un-powered resistive sheet, the sensitivity to noise is reduced and system performance is improved.
In U.S. Pat. No. 5,041,701 (Wolfe et al.) is described an edge linearization device for producing orthogonal electric fields in a resistive surface to be used in a contact input system. The system is capable of locating an object in contact with the resistive surface. In U.S. Pat. No. 5,083,118 (Kazama) is described a coordinate measuring apparatus to be mounted to a CRT or an LCD screen in which a fist layer is used as an electromagnetic screen and resistors of value approximately one hundred ohms are connected to one end of the resistive layers to help reduce noise. Shown in U.S. Pat. No. 5,191,175 (Protheroe et al.) is shown a self tuning digitizer control circuit in which a narrow bandpass filter is used to filter out extraneous noise picked up by a coordinate sensing stylus. In U.S. Pat. No. 5,365,253 (Cheng et al.) is shown a digitizer device with anti-noise capability. The digitizer operates at several hundred thousand Hertz and uses a hardware circuit controlled by a software program to eliminate outside noises.
Resistive digitizers have two resistively coated surfaces that come into contact when pressed. A standard four wire technique involves applying a voltage across one of the surfaces while reading the voltage at the point of contact through the other layer. A controller switches between the two resistive surfaces and measures the voltage corresponding to the X and Y coordinates. The noise induced into the resistive surfaces when the digitizer is placed on a CRT or an LCD screen can produce errors in the coordinate readings.